Thin films of metal are frequently fabricated upon solid substrates for use in the electronics and opto-electronics industries by what is termed a Metal Organic Chemical Vapor Deposition (MOCVD) process. The process typically comprises introducing the vapor of at least one metal-organic compound, i.e., the precursor of the metal, into a reactor under conditions of temperature and pressure such that the precursor decomposes to give a deposit of the metal on a solid substrate contained within the reaction chamber. The deposit is typically obtained as a thin film of the metal on the substrate. In the deposition of what is termed III-V semiconducting materials, vapors comprising at least one element of Group III of the Periodic Table are mixed with vapors of at least one element of Group V of the Periodic Table and the resulting thin film deposited by the process are a III-V semiconducting material or alloys of the Group III and Group V metals. Examples of III-V materials which are prepared by such a method include GaAs, AlAs, InP and alloys thereof, e.g., materials such as Al.sub.x Ga.sub.1-X As, Ga.sub.y In.sub.1-y P and Ga.sub.X In.sub.1-X As.sub.y P.sub.1-y wherein x and y independently are in the range of zero to 1 inclusive. These materials are commonly referred to as ternary and quaternary alloys. It should be understood that the above III and V designations refer to the elements of Groups 3a and 5a of the Periodic Table of Elements of the Handbook of Chemistry and Physics, 63rd Ed.
For the fabrication of devices of specific electronic properties, it is necessary to precisely control the composition and the thickness of the layer being deposited by the MOCVD process. This control in turn requires the careful control of the metal precursor vapors entering the reaction chamber. The manner by which the latter control is achieved depends in part upon the physical state of the metal precursor at ambient temperature and one atmosphere pressure. For example, metal precursors which are gases at those conditions are often stored in a high pressure cylinder and the dosimetry of the metal precursor, in gaseous form, to the reactor is controlled by a calibrated mass-flow controller. In contrast, solid and liquid precursors are usually placed in a vessel equipped with an inlet and an outlet so that a carrier gas such as hydrogen, helium, argon or nitrogen can be passed through the vessel to entrain vapors of the metal precursor and conduct the vapors to the reactor.
The vessel typically employed in MOCVD is known as a "bubbler" and is a vessel fitted with an inlet tube which conducts carrier gas to the bottom of a sample of metal precursor material by means of a dip-tube, and an outlet which opens directly from the top of the vessel. The carrier gas thereby entrains some of the vapors of the metal precursor before leaving the bubbler at its outlet.
When a bubbler is employed to control the dosimetry of metal precursors into an MOCVD reactor, accurate control of the carrier gas is achieved through the use of a mass-flow controller. However, the entrainment of solid or liquid metal precursor vapors depends upon the contact between the carrier gas and the metal precursor as well as the rate at which the carrier gas becomes saturated with metal precursor vapor. Some metal precursors readily and rapidly saturate the carrier gas. Other materials, e.g., solid arsenic, do not readily vaporize and carrier gas saturation is difficult and slow. An additional common problem is the tendency of many solid organometallic compounds to react with traces of oxygen or water in the carrier gas to form an oxide skin over the non-gaseous phase which retards vaporization and again makes saturation of the carrier gas difficult.
Reproducible dosimetry from a conventional MOCVD bubbler is obtained if saturation of the carrier gas with metal precursor vapor under the conditions employed is ensured and maintained. Under typical operating conditions of temperature and pressure, metal precursors which are liquid will approach saturation of the carrier gas to an extent which will provide reproducible dosimetry of metal precursor vapor from a conventional bubbler until the liquid metal precursor has essentially been exhausted, or at least for so long as the entire carrier gas is able to bubble through the liquid metal precursor.
For solid metal precursors, however, the concentration of metal precursor vapor in the carrier gas is highly variable and typically fluctuates throughout the usage of the precursor. The fluctuation of metal precursor concentration is considered to be likely due at least in part to changes in the surface of the solid being employed as metal precursor.
One of the more difficult metal precursors to employ in a MOCVD process is trimethylindium, (CH.sub.3).sub.3 In. This material is a crystalline solid melting at 89.degree. C. and is by far the most commonly used metal precursor for the deposition of indium. The use of trimethylindium becomes particularly difficult when it is desired to provide a metallic layer of the same crystalline structure as that of the solid substrate. Even small variations in the dosimetry of vapors resulting from trimethylindium produce undesired and unsatisfactory results.
Various attempts have been made to improve deposition of a metal from solid metal precursors such as trimethylindium. One such attempt utilized triethylindium as a replacement. However, although this latter compound is a liquid at ambient temperature, it has a vapor pressure considerably lower than trimethylindium and is less thermally stable. In part because of these factors, triethylindium does not produce uniform deposition over the surface of the substrate. Other indium-containing liquid such as EQU (CH.sub.3).sub.3 In.NH[CH(CH.sub.3).sub.2 ].sub.2, ##STR1## EQU (CH.sub.3).sub.3 In.P(CH.sub.2).sub.2 (CH.sub.2 CH.sub.3)
have also been evaluated as a metal precursor but each of these compounds has a vapor pressure below that of trimethylindium and the resulting rate of growth of indium deposit is undesirably slow.
Mechanical variations have also been used to attempt to improve the efficiency of trimethylindium use, for example, by reversing the flow of carrier gas through the bubbler. In published Japanese application 01-265511 the trimethylindium precursor is deposited on spherical supports. It has also been suggested to melt the trimethylindium prior to use to provide more uniform surface of the sample. In an article by Butler et al, "Variations in TMI Partial Pressure Measured by Ultrasonic Cell on a MOVPE-Reactor", J. of Cr. Gr., 94, pp. 481-487 (1989), it is suggested to monitor the composition of the carrier gas leaving the bubbler maintained at constant temperature. The flow of carrier gas is then adjusted to compensate for any increase or decrease in the efficiency of the collection of metal precursor vapor. Adducts of trimethylindium and organic compounds such as alkylphosphines are used in U.S. Pat. No. 4,716,130. Another approach is to modify the conventional bubbler with a special diffuser and enclosing the metal precursor within a porous membrane. Such apparatus provides more reliable dosimetry but tends to become plugged through formation of oxides by reaction with impurities in the carrier gas.
Each of the above methods suffers from some disadvantage which causes the procedure to be more complex and/or less economical. It would be of advantage to provide a method for conducting a MOCVD process in which a solid metal precursor is used in a conventional bubbler but in which a reproducible dosimetry of entrainment of metal precursor vapors is obtained.